Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.
During installation, one or more blades are to be mounted to the wind turbine hub. The hub generally comprises a plurality of annular mounting flanges with openings and/or a plurality of surfaces prepared for attachment, lifting or manipulation. The blade can comprise a plurality of fasteners, such as bolts, or pins or studs at its blade root. During installation, these fasteners are to be fitted into the openings in the mounting flanges.
The blades may be installed in a variety of ways, e.g. in a substantially vertical manner or substantially horizontally or at other inclined angles. Particularly, in offshore wind turbines, mounting a blade from above the hub may be very complicated, or at least require an enormous crane and jack-up barge. Such an installation method may thus be very expensive and depending on the depth of the sea, the blade size and hub height may simply be impossible.
It is known to hoist a complete rotor assembly, i.e. the hub with blades, and mount it to e.g. the nacelle. But in order to mount a complete rotor assembly, a large surface area is required, which is typically not available in the case of offshore wind turbines.
It is further known to mount an incomplete rotor assembly on the nacelle, e.g. the hub with two blades and subsequently, mount the remaining blade. In these cases, the rotor with the two blades is normally mounted with the two blades pointing upwards, i.e. “bunny ears” configuration. There is thus no need for rotating the wind turbine rotor as the third blade can be vertically mounted from below. However, in order to be able to perform these operations, the prevailing wind speed has to be below a predetermined value for a prolonged period time. The period of time depends on the expected length of the installation step and a safety factor to be taken into account.
During certain seasons, windows of several hours in which the wind does not reach the predetermined value may not be available very often. In practice, this may mean that personnel and maintenance equipment including e.g. expensive cranes and jack-up barges may be in stand-by during days or even weeks. This can represent an enormous cost.
It is known to mount each of the plurality of blades substantially horizontally or substantially vertically. This means that individual installation steps may require less time and/or may be performed at higher winds, thus increasing the time windows available for installation. However, such methods require rotation of the hub between the mounting of one blade and another. In order to correctly position the hub, torque is required for rotating the wind turbine rotor after mounting one blade in order to mount the next one. When not all blades have been mounted onto the hub, the hub is not balanced, so that the weight of one or more blades has to be carried upwards when rotating the hub. The corresponding torque may thus be very high.
The torque may be delivered using the gearbox (when present) with an external power supply for rotating the rotor. Such a system may be used in combination with a locking pin for maintaining the rotor in a desired position for installation. This is not possible in case of directly driven wind turbines as no gearbox is provided between rotor and generator. Even though the generator may be suitable for operating as a motor, it will generally not be able to deliver the required torque to rotate the hub, especially when the hub is unbalanced, i.e. when at least one blade, but not all the blades have been mounted.
It is an object of the present disclosure to provide methods and systems that at least partially reduce one or more of the aforementioned drawbacks leading to improved methods of installing wind turbine blades especially suitable for direct drive wind turbines and offshore direct drive wind turbines.